/Engine/Source/Runtime/Core/Public/IO/IoDispatcher.h
Quick synchronous existence check that returns the number of blocks for the chunk. This is used to set up the necessary structures without needing to read the source data for the chunk.
bool ChunkExists ( const TPair< [FIoContainerId](API\Runtime\Core\IO\FIoContainerId), [FIoChunkHash](API\Runtime\Core\IO\FIoChunkHash) > & InChunkKey, uint32 & OutNumChunkBlocks )